This invention relates to a device for detecting an actual air/fuel ratio of an air-fuel mixture subjected to combustion in a combustor, such as the combustion chambers of an internal combustion engine, based on the magnitude of an oxygen partial pressure in the combustion gas exhausted from the combustor.
Recently a marked tendency to electronical control of internal combustion engines has been shown, particularly in the automobile industries, with a primary object of achieving an extremely minute control of the air/fuel ratio thereby improving the fuel economy and further reducing the emission of HC, CO and Nox by, for example, enhancing the efficiency of a three-way catalyst. In many cases an electronically controlled engine system includes an oxygen sensor to detect the concentration of oxygen in the exhaust gas as an indication of an actual air/fuel ratio of an air-fuel mixture supplied to the engine. For example, a feedback signal provided by the oxygen sensor is put into an electronic control circuit which provides a control signal to an electromagnetic flow control valve for minute control of the feed rate of either fuel or auxiliary air.
Oxygen sensors prevailing for this purpose are of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte, such as ZrO.sub.2 stabilized with CaO, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side. These oxygen sensors are designed and used such that a reference oxygen partial pressure is maintained on the reference electrode side by using a certain oxygen-containing substance, while the measurement electrode layer is exposed to the exhaust gas. When there occurs a change in the air/fuel ratio of an air-fuel mixture supplied to the engine across the stoichiometric air/fuel ratio, a great and sharp change is exhibited in the magnitude of an electromotive force the oxygen sensor in the exhaust gas generates. Accordingly this type of oxygen sensor is suitable to applications to engines operated with a stoichiometrical or nearly stoichiometrical air-fuel mixture. However, it is impossible to detect or estimate air/fuel ratio values of either a lean mixture or a rich mixture by the use of an oxygen sensor of this type in the exhaust gas because, when the air/fuel ratio varies but remains on one side of the stoichiometric ratio, the magnitude of the electromotive force exhibits only very small changes, if not changeless, compared with changes in the air/fuel ratio.
Meanwhile the development of so-called lean-burn engines has been in progress with the view of attaining a maximal thermal efficiency together with advancement of the exhaust emission control. Also, so-called richburn engines have attracted attention because of the possibility of achieving a very high mechanical efficiency. Accordingly there is a keen demand for an oxygen sensor which is to be used in exhaust gases (since it is more convenient to provide an oxygen sensor to the exhaust system of an engine than to the intake system) and enables to detect air/fuel values deviated from a stoichiometric ratio.
In our prior U.S. Patent Application Ser. No. 28,747 filed on Apr. 10, 1979 and now U.S. Pat. No. 4,224,113, we have proposed a method of detecting an actual air/fuel ratio of an air-fuel mixture, which may be either a lean mixture or a rich mixture, subjected to combustion in, for example, an internal combustion engine by disposing an oxygen sensitive probe, which is of the concentration cell type having a solid electrolyte layer sandwiched between a measurement electrode layer and a reference electrode layer which is covered with a shield layer, in the exhaust gas and forcing a constant DC current of an adequately predetermined intensity to flow through the solid electrolyte layer between the two electrode layers in a selected direction with the intention of constantly maintaining an appropriate magnitude of reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer by causing migration of oxygen ions through the solid electrolyte layer in a selected direction. (The particulars of this oxygen sensing probe will be described hereinafter.) Either air/fuel ratios above a stoichiometric ratio inclusive or air/fuel ratios below the stoichiometric ratio inclusive can be detected (depending on the direction of the flow of the DC current) by this method, but in the case of an engine or a combustor being operated sometimes with a rich mixture and at other times with a lean mixture it is impossible to detect every air/fuel ratio value realized in the engine by this method.